The present invention relates generally to an improved method of medical imaging over large areas, and more particularly, to a method and apparatus of acquiring magnetic resonance (MR) images over an area that is greater than the optimal imaging area of an MR scanner using continuous table movement through the MR scanner without incurring slab-boundary artifacts.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In such MRI systems, the volume for acquiring MR data with optimal gradient linearity, having a uniform magnetic field Bo, and uniform radio frequency (RF) homogeneity is of limited extent. Desired fields-of-view (FOV) that exceed this limited volume are traditionally acquired in sections, with table motion between scans. The resulting concatenated images often exhibit discontinuities at the slab junctions. These slab-boundary artifacts result in non-ideal images. When these artifacts are either severe or occur in a critical region-of-interest, complete re-acquisition of data may be needed for a thorough analysis. In addition, the time for table motion extends scan time beyond that required for data acquisition.
Known methods designed to eliminate slab boundary artifacts in angiograms and 3D fast spin echo acquisitions include Sliding Interleaved ky Acquisition (SLINKY) and Shifted Interleaved Multi-Volume Acquisition (SIMVA). These methods however, move the slab-selective excitation while keeping the table stationary rather than moving the table and keeping the slab position stationary. As a result, these methods are limited by the inherent optimal imaging volume of the MR scanner. These methods employ phase encoding in the z-direction and do not account for those situations where the z matrix is not equal to the number of kx-ky subsets. Similarly, other known methods designed to eliminate slab boundary artifacts have likewise moved the imaging slab rather than the patient table. Other known techniques implement a continuously moving table in both the phase encoding and frequency direction. However, prior techniques that move the patient table in the frequency-encode direction encode the data as if they were acquiring an image of the entire FOV and data is combined prior to any Fourier transform.
Other known systems employ stepped table and/or moving table approach with an array of receiver coils that move with the imaged object. Data is collected from each coil independently as it moves through the homogeneous volume of the scanner. None of these known systems however, collect data from a slab thickness that is fixed relative to the magnet, place the frequency encoding axis in the direction of table motion, and combine the data after Fourier transforming in the direction of table motion.
It would therefore be desirable to have a new method and apparatus that allows coverage of large FOV without slab-boundary artifacts in the resulting concatenated images.